WASHINGTON — Senator John McCain, the 2008 Republican presidential nominee known for his independent streak over more than three decades representing Arizona in the Senate, has brain cancer, his office disclosed Wednesday night in a statement from the Mayo Clinic.
The statement said the medical condition was discovered after Mr. McCain, 80, who was re-elected to a sixth term in November, underwent surgery to remove a blood clot above his left eye at the Mayo Clinic Hospital in Phoenix.
“Subsequent tissue pathology revealed that a primary brain tumor known as a glioblastoma was associated with the blood clot,” the statement said. “Treatment options may include a combination of chemotherapy and radiation.”
Medical experts said his condition was extremely serious. Eugene S. Flamm, chairman of neurosurgery at Montefiore Hospital in the Bronx, said that “a glioblastoma is the most common and most malignant of brain tumors.”
The median survival is about 16 months, he said. Senator Edward M. Kennedy, Democrat of Massachusetts, died in 2009 from this kind of aggressive brain tumor, which almost always grows back even after chemotherapy and radiation.
The illness of Mr. McCain, a former Navy pilot who was captured and held as a prisoner during the Vietnam War, had implications this week for the health care debate. His absence caused Senator Mitch McConnell, Republican of Kentucky and the majority leader, to postpone a floor fight until Mr. McCain returned to Washington.
The diagnosis shook the Senate, where Mr. McCain is a popular figure despite his occasionally heated disputes with colleagues in both parties. About a dozen lawmakers who on Wednesday night were gathered in the Dirksen Senate Office Building to explore a health care compromise asked Senator James Lankford, an Oklahoma Republican who is a Baptist minister, to lead them in prayer for their colleague.
“It was very emotional,” said Senator John Hoeven, Republican of North Dakota, who was present. He said finding a solution to the health care impasse was “more challenging without him.”
Mr. McConnell called Mr. McCain a hero to both Senate Republicans and the nation at large.
“He has never shied from a fight, and I know that he will face this challenge with the same extraordinary courage that has characterized his life,” Mr. McConnell said Wednesday night. “We all look forward to seeing this American hero again soon.”
The disclosure on Wednesday suggested that Mr. McCain’s condition was more serious than initially believed, although the statement said that “he is recovering from his surgery ‘amazingly well,’” according to his doctors, “and his underlying health is excellent.”
President Trump on Wednesday night issued a statement urging Mr. McCain to quickly recover.
“Senator John McCain has always been a fighter,” Mr. Trump said.
As the Republican health care bill teetered in recent days, Mr. Trump referred affectionately to Mr. McCain as a “crusty voice” in the capital. But their relationship has been fraught. In July 2016, during the presidential campaign, Mr. Trump denigrated Mr. McCain’s military record, saying he was considered a war hero only because he was captured. “I like people who weren’t captured,” Mr. Trump said in a comment that many suggested had gone too far. Mr. McCain, who at the time was facing conservative opposition in his re-election to his Senate seat, remained a supporter of Mr. Trump throughout the campaign.
However, Mr. McCain has pressed for a deep investigation into any interference into the presidential campaign that was conducted by Russia, which he considers an adversary. On Tuesday, and despite his condition, Mr. McCain issued a statement warning about Russian intrusion in Ukraine and calling for new sanctions against Moscow.
On Twitter, former President Barack Obama, who defeated Mr. McCain in the 2008 election, called the senator a “hero & one of the bravest fighters I’ve ever known. Cancer doesn’t know what it’s up against. Give it hell, John.”
Mr. McCain, who had failed to win his party’s presidential nomination in 2000, faced an uphill battle in 2008 given public discontentment with the United States-led war in Iraq and overall fatigue with the administration of President George W. Bush. Mr. McCain picked Sarah Palin, then the governor of Alaska, as his vice-presidential running mate in an effort to reinvigorate a flagging campaign with a woman on the ticket.
A crucial moment in the race occurred in late September 2008 when he suddenly suspended his campaign and returned to Washington for an economic summit meeting at the White House that was also attended by Mr. Obama, then a Democratic senator from Illinois. Mr. McCain’s 11th-hour effort to focus on the economy was generally considered a lost opportunity, and the financial collapse helped propel Mr. Obama to the presidency.
Mr. McCain currently leads the Senate Armed Services Committee and is a top proponent of using military force overseas. The Senate is preparing to take up the annual Pentagon policy measure produced by the committee.
In Congress, he is probably best known for his efforts to champion changes in campaign finance laws over the fierce objections of some of his Republican colleagues, particularly Mr. McConnell.
Mr. McCain’s office said he “appreciates the outpouring of support he has received over the last few days.” The statement added: “He is in good spirits as he continues to recover at home with his family in Arizona. He is grateful to the doctors and staff at Mayo Clinic for their outstanding care, and is confident that any future treatment will be effective. Further consultations with Senator McCain’s Mayo Clinic care team will indicate when he will return to the United States Senate.”
The standard of care includes surgery to remove as much of the tumor as possible, and then radiation and chemotherapy. In the past, radiation sometimes diminished patient’s brain function, but techniques have improved so that more healthy brain tissue is spared and patients fare better, said Dr. Mitchel S. Berger, a neurosurgeon and glioblastoma expert at the University of California, San Francisco.
Senator Jeff Flake, the junior Republican senator from Arizona, acknowledged the seriousness of Mr. McCain’s diagnosis in a still-hopeful Twitter post. “Tough diagnosis, but even tougher man,” Mr. Flake wrote.